AbstractTheppd1mutant of yeast,Saccharomyces cerevisiae, was isolated as a suppressor of thecyr2mutation which caused alteration of the catalytic subunit of cAMP‐dependent protein kinase. Three peaks of phosphoprotein phosphatase activity (peak I, II and III) were identified by DEAE‐Sephacel chromatography of crude extracts of the wild‐type strain. Theppd1mutant was deficient in peak III phosphoprotein phosphatase activity. The peak III enzyme efficiently utilized the phosphorylated forms of NAD‐dependent glutamate dehydrogenase and trehalase as substrate. Theppd1mutation did not suppress thecyr1,CYR3orras1 ras2mutations. Theppd1locus was located on chromosome II and had identical characteristcs withglc1. Theppd1mutation suppressed the G1 arrest caused by nutritional limitation, but maintained sensitivity to mating pheromone. In diploids homozygous for theppd1mutation, no premeiotic DNA replication and commitment to intragenic recombination occurred and no spores were formed, suggesting that the accumulation of phosphorylated proteins in the absence of one of the phosphoprotein phosphatases is required for mitosis but not for the initiation of